The present relation relates to a process for batchwise or continuous preparation of tetrahydrofuran polymers by homopolymerization of tetrahydrofuran or copolymerization of tetrahydrofuran with 1,2-alkylene oxides in the presence of acid-activated and calcined aluminium silicate, in which process homo- and copolymers are obtained which have a particularly narrow molecular weight distribution, contain a negligible amount of oligomeric cyclic ether contaminants and have low color and acid numbers.
The catalytic synthesis of tetrahydrofuran polymers on aluminium silicates is well known. British patent No. 845 948 describes a process for copolymerizing tetrahydrofuran with alkylene oxides, in which the polymerization is carried out in the presence of compounds with reactive hydrogen and bleaching-earth catalysts. In this process, relatively large amounts of low-molecular-weight products consisting predominantly of oligomeric cyclic ethers are obtained as byproducts. The molecular weight distribution of the tetrahydrofuran polymers is very broad. Depending on the average molecular weight, it can assume a polydispersity M.sub.w /M.sub.n of 3 to 4 for the molecular weight range from 1000 to 2000 (M.sub.w : weight-average molecular weight; M.sub.n : number-average molecular weight). The polymers are altogether yellowish in color, are for the most part not strictly bifunctional and have an acid number (usually &gt;0.1 mg KOH/g) which makes them unsuitable for further processing to polyesters and polyurethanes.
The homopolymerization of tetrahydrofuran to polybutylene glycol diacetates using montmorillonite silicates as catalysts is described in the German published examined application No. 1226560. Powdered catalysts prove superior to granular ones (cf. column 4, lines 34-36). This process, however, results in products which alone on account of their poor color require costly purification if they are to be processed further. These problems are described in the unexamined German laid open patent application No. 3935750.
The oligomeric cyclic ethers formed during the homo- and copolymerization reactions are undesirable contaminants in the polymers since they represent inert material and lower the quality of the final polymers made from polymeric glycols. Various proposals for reducing their content have already been made. For example, it is proposed in the EP-A 6107 that the copolymers be treated at an elevated temperature with an activated montmorillonite. In the US Patent No. 4127513 it is suggested that a specially activated montmorillonite be used as catalyst. The disadvantages of this process are the high color numbers of the polymers, the relatively low rate of polymerization and an oligomeric cyclic ether content which is still as much as 5 to 6% by weight.
A further proposal for improving the copolymerization of alkylene oxides with tetrahydrofuran under the catalytic action of activated bleaching earth is to be found in the U.S. Pat. No. 4,228,272, which describes the use of bleaching earths with a given specific pore volume, a defined catalytic surface area and a given pore diameter. The oligomer content of 4 wt. % (cf. column 5, lines 14-15) is, however, still too high for the copolymers to be used in the production of polyurethanes, on which higher mechanical demands are made.
As is known, all processes for copolymerizing alkylene oxides with tetrahydrofuran in the presence of bleaching earths result in high-molecular weight copolymers with terminal hydroxyl groups, the copolymers always being contaminated by a variable proportion of macrocyclic ethers without hydroxyl groups. Hence it has also been proposed that the cyclic ethers be removed by way of extraction with hydrocarbons, water or supercritical gases (U.S. Pat. Nos. 4,500,705, 4,251,654 and 4,306,056).
The unexamined German laid open patent application 3 3 46 136 describes a process for the copolymerization of alkylene oxides and tetrahydrofuran where the formation of cyclic oligomeric ethers is prevented from exceeding 2 wt. % by polymerizing continuously in a reactor and adding to the circulating reaction mixture less than 30 wt. % of fresh feed comprising tetrahydrofuran and 1,2-alkylene oxide. The disadvantage of this process is that the resulting polymers have a broad molecular weight distribution, and the polydispersity M.sub.w /M.sub.n exceeds 4 in this case.
From the EP-A-0104 609 it is known that polyoxybutylene oxyalkylene glycolic acid diesters with a low oligomeric cyclic ether content are obtained if the copolymerization of tetrahydrofuran and a 1,2-alkylene oxide is carried out in the presence of carboxylic acid anhydride and bleaching earth with a water content of less than 3 wt. %. However, in this process too, the molecular weight distribution of the copolymers is unsatisfactory. The two recently developed methods of polymerizing with bleaching earths or amorphous aluminium silicates (U.S. Pat. Nos. 5,208,385 and 5,210,283) are not satisfactory either. Only very heterogeneous polymers are obtained, which have the further disadvantages of being colored and having acid numbers &gt;0.1 mg KOH/g.
The EP-A-0239787 contains a proposal for narrowly limiting the molecular weight distribution and reducing the formation of oligomeric cyclic ethers by means of batchwise copolymerization using bleaching earth catalysts in the presence of telogenes with reactive hydrogen atoms and by keeping the concentration of 1,2-alkylene oxide constant but low.
In comparison to these products, there is nevertheless still a need to reduce the molecular weight distribution of both homo- and copolymers with molecular weights between 1000 and 2000 to a value even lower than 1.5 to 2.8, for example to a value between 1.3 and 2.2, and to limit the formation of oligomeric cyclic ethers to less than 2 wt. %. In the desired process low-molecular-weight polymers with an average molecular weight of less than 250 should not be formed at all, or at the most to an extent of 1 wt. %. An additional purification step to remove these products, as is required with the processes currently in use, would then be unnecessary.